Dilute combustion of gasoline in an internal combustion engine, using either air or recirculated exhaust gas (EGR), is known in the art to enhance the thermal efficiency and decrease the production of oxides of nitrogen (NOx). However, there is a limit to which an internal combustion engine may operate with a dilute mixture due to misfire and combustion instability resulting from a slow burn rate of the charge mixture. Known methods to extend the dilution tolerance limit include: 1) improving the ignitability of the mixture by enhancing ignition and mixture preparation, 2) increasing the flame speed by introducing charge motion and turbulence, and 3) operating the internal combustion engine in a controlled auto-ignition combustion mode.
The controlled auto-ignition process may be referred to as Homogeneous Charge Compression Ignition (HCCI). In this process, a charge mixture of EGR, air, and fuel is created and auto-ignition is initiated simultaneously from multiple ignition sites within the compressed charge mixture, thereby resulting in stable power output and high thermal efficiency. Since the combustion is highly dilute and uniformly distributed throughout the charge mixture, the temperature of the products of combustion is typically lower than that of a traditional spark ignited internal combustion engine with a propagating flame front and the diesel engine with an attached diffusion flame. The reduced temperature of the products of combustion may result in reduced NOx emissions when operating in the HCCI mode. Known methods to induce controlled auto-ignition at part load include: 1) heating the intake air, 2) varying the compression ratio, and 3) blending gasoline with fuels that have wider auto-ignition ranges than that of gasoline. In all the above methods, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow.
A four-stroke direct injection internal combustion engine may operate in the HCCI mode by employing various valve opening and closing strategies. By altering the valve profiles, or operating characteristics of the exhaust valves and/or the intake valves, a high proportion of residual products of combustion may be retained within the cylinder of the internal combustion engine to provide favorable conditions to auto-ignite a highly dilute charge mixture. The range of engine speed and load over which controlled auto-ignition combustion can occur may be expanded by employing various valve operating strategies, thereby obviating the need to increase the compression ratio of the spark ignited (SI) internal combustion engine.
One such valve strategy is exhaust re-compression. With this strategy, the exhaust valve is closed earlier in the exhaust stroke than in a typical four-stroke internal combustion engine. Correspondingly, the intake valve is opened later than in a typical four-stroke internal combustion engine. The early exhaust valve closing and late intake valve opening provides a negative valve overlap period where products of combustion become trapped within the engine's cylinder. These trapped products of combustion will mix with and warm the fuel and air charge mixture during the intake stroke of the internal combustion engine, thereby promoting the auto-ignition process.
Another valve strategy is exhaust re-breathing. With this strategy, the exhaust valve is opened for a first period to allow combusted gasses to be expelled from the combustion chamber. Subsequently, the exhaust valve opens for a second period to allow products of combustion previously exhausted to be drawn back into the cylinder. By opening the exhaust valve twice during each cycle of the four-stroke internal combustion engine, favorable conditions are created within the combustion chamber to support stable auto-ignition combustion.
Yet another valve strategy is a hybrid between exhaust re-compression and exhaust re-breathing. In this mode, the exhaust re-compression mode may be used when the internal combustion engine is operating at a low engine load. For higher engine loads, the exhaust re-breathing strategy may be used. Additionally, by varying the exhaust valve lift and intake valve timing, the spark-ignited internal combustion engine may operate in a non-throttled load control mode (NTLC). In this mode, the intake valve timing will vary the engine load by controlling the amount of intake air communicated to the cylinder.
To enable the above-mentioned operating strategies, the internal combustion engine may be equipped with a variable valve actuation (VVA) system ranging from cam phasers and two-step valve actuation to a fully flexible valve actuation (FFVA) system.